The most tantalizing prospect in dropping the 4th color, to me, is the new opportunity for a spot varnish plate. Maybe texture varnish the hi-lites on the curtain, or even over the black in the film frames.
Back to the profit margin:
Spot varnish is gorgeous, but it's pointless on hi-gloss papers (eg Kromkote).
The client will very likely elect spot varnish on a cheaper matte paper, as opposed to flat print on expensive gloss papers.
Bottom Line:
You get the benifits of a fifth color, without the setbacks of a 5-color press or an overprint run.
Pricey gloss is ruled out, leaving you a choice of either a better-quality matte paper or more dollars in your pocket.